1. Field of the Invention
The present invention relates to an image display device having a backlight capable of controlling light intensity.
2. Related Art
In recent years, a technique to modulate the backlight luminance of a liquid crystal display in accordance with video signals has been studied in order to improve the contrast of the image to be displayed and to reduce power consumption.
When modulating the backlight luminance, liquid crystal transmittance has to be corrected in accordance with the luminance of backlight emitted to a liquid crystal panel in order to maintain the luminance of the image to be displayed. When the backlight luminance value is set low, there is a case where a gray-scale level value which has been corrected in accordance with a backlight luminance value to control the liquid crystal transmittance exceeds a displayable value of the liquid crystal panel. Therefore, in various disclosed techniques, when the gray-scale level value corrected in accordance with the backlight luminance value exceeds the displayable value of the liquid crystal panel, the gray-scale level value of a corrected image exceeding a displayable range is corrected to be the maximum displayable value, or a rounding gray-scale level correction process (see JP-A 2004-325628 (Kokai), for example) is performed. However, in these techniques, there is a problem that the color tone of the image to be displayed is drifted for the input video signal.
Further, a method to prevent the color drift between the color tone of the display image and that of the input image has been investigated (see JP-A 2003-99010 (Kokai), for example). Such a method includes the steps of: detecting the maximum peak level from the peak levels of RGB colors of the input image signal; calculating an image gain based on the maximum peak level; amplifying the input image signal in accordance with the image gain; and modulating the backlight luminance in accordance with the image gain. In these techniques, all signals of the input image are amplified at one time in accordance with the image gain while the luminance of the backlight is modulated. Accordingly, the backlight has to emit light having the determined luminance level to liquid crystal panel wholly and equally in order to display an image having desired luminance and color. However, when a plurality of backlights are arranged in each area, the emission distribution of backlight in the screen is not equalized and an image having desired luminance and color cannot be displayed. Further, since the backlight luminance is set in accordance with the maximum peak level of the input signal, the backlight luminance tends to be set at a brighter level. In such a case, sufficient contrast cannot be obtained.
In the above conventional techniques, when the gray-scale level value of transmittance exceeds the displayable range of the liquid crystal panel, the gray-scale level values of RGB subpixels are corrected independently of one another. Therefore, each subpixel has different correction gain, and color drift is caused between the input image and the output image.